Certain compounds in the exhaust stream of a combustion process, such as the exhaust stream from an internal combustion engine, are undesirable in that their release into the environment would contribute to lower air quality. As a result, they must be controlled in order to protect the environment and to meet or exceed government emissions regulations. Among such undesirable compounds are nitrogen oxides, which will be referred to as NOx. There are a wide variety of combustion processes that produce NOx, such as coal- or oil-fired furnaces, reciprocating internal combustion engines (including gasoline and diesel engines), and gas turbine engines. In each of these combustion processes, control measures to prevent or diminish atmospheric emissions of NOx are needed in order to improve air quality and comply with governmental regulations.
In order to remove pollutants (e.g., NOx, CO, and hydrocarbons) from an exhaust stream of an engine, catalytic converters containing three-way catalysts have been installed in automobiles. For most of the light-duty vehicles in US, the vehicle engines operate in a stoichiometric mode; i.e. stoichiometric amounts of fuel and air are fed into the engine and after combustion the exhaust stream contains roughly equal amount of residual oxygen and reductants (e.g., hydrocarbons, CO, and H2). In these exhaust stream, the three-way catalysts are able to reduce NOx to N2, and oxidize CO, H2, and hydrocarbons to CO2 and H2O. However, in a fuel-rich exhaust, CO and hydrocarbons cannot be completely oxidized to CO2 and H2O due to an insufficient amount of oxygen present in the exhaust. On the other hand, in a fuel-lean exhaust, NOx cannot completely reduced to N2 due to an insufficient amount of reductants and an excess of oxygen in the exhaust.
Lean-burn engines typically operate in a fuel-lean mode; i.e. a more than stoichiometric amount of air is fed with fuel into the engine cylinders. Compared with stoichiometric engines, lean-burn engines offer superior fuel economy. One typical example of a lean-burn engine is a diesel engine. However, the fuel-lean exhaust stream from the lean-burn engines render the three-way catalytic converters inadequate in converting NOx to N2 and reducing tailpipe NOx emissions due to the reason mentioned above. Lean NOx traps, sometimes called LNT's, are considered to be one of the leading technologies for removing NOx from the exhaust stream of lean-burn engines. They contain catalysts that reduce NOx to N2 and compounds (such as metal nitrates) that can store NOx as nitrates. However, LNT's operate most effectively within a temperature range of approximately 250-550° C., and undesirable emissions of NOx into the atmosphere are more likely to occur at temperatures outside that range. At temperatures below about 250° C., such as in an engine cold start, the LNT does not effectively convert NOx to nitrates (for storage) in a fuel-lean mode or convert released NOx to N2 in a fuel-rich mode. When the LNT temperature is above approximately 550° C., the metal nitrates are not stable enough to store the NOx.
Therefore, despite efforts to develop a device or system effective for reducing NOx to nitrogen in a lean-burn engine (such as a diesel engine), the need for adequate conversion effectiveness has remained unsatisfied. Moreover, there is a continuing need for improved effectiveness in treating NOx emissions from any combustion process, but especially during an engine cold start.